1. Field of Invention
The present invention relates to a mobile communication system, a wireless base station apparatus and mobile terminal provided in the mobile communication system, and a control method therefor.
2. Description of the Related Art
In recent years, the amount of data traffic is expected to greatly increase not only for a fixed radio communication network but also for a mobile radio communication network. Accordingly, HSDPA (High Speed Downlink Packet Access) was proposed in Release 5 of 3GPP (3rd Generation Partnership Project) for W-CDMA (Wideband Code Division Multiple Access), HSDPA enables enlarged data traffic in a downlink circuit (radio link from a wireless base station apparatus to a mobile terminal).
The HSDPA employs HARQ (Hybrid Automatic Repeat reQuest) as an error correction technique. Moreover, according to HSDPA, the downstream data for a number of mobile terminals are multiplexed with respect to time and code into one common channel. For each mobile terminal using the common channel, transmission opportunities are scheduled based on a radio link quality. In a mobile communication system employing HSDPA, the packets are transmitted to a number of mobile terminals under various conditions through the common channel from the wireless base station apparatus, thus, providing an improvement in the system throughput with the larger number of mobile terminals.
In the wireless base station apparatus utilizing the HSDPA as described above, since a spreading encoder provided for each cell is shared in the communication with a plurality of mobile terminals, the transmission signal from the spreading encoder should be allocated among the transmission data sent to the mobile terminals according to priority (degree of priority for transmission opportunity), which is set for each mobile terminal. An example of a wireless base station apparatus enabling the priority control described above is in the 3GPP TR25.321 V5.7.0 (2003-12) “MAC protocol specification”, which discloses that a wireless base station apparatus comprises a buffer unit for temporarily holding the user data for each mobile terminal.
Furthermore, in the mobile communication system where mobile terminals establish communications while moving, a buffer control, as well as priority control, is needed in order to securely transmit the user data to the mobile terminal without loss of data. The buffer control is needed even if the mobile terminal changes its associated cell while moving among the cells.
For example, in a mobile communication system where the buffer unit corresponding to each of a plurality of cells managed by the wireless base station apparatus is provided, all user data remaining in the buffer unit corresponding to a certain cell is completely transmitted while the mobile terminal is associated with the certain cell. If the mobile terminal moves to another cell before completing the transmission of the user data because the mobile terminal is moving fast, the transmission of the user data in the buffer unit may not be completed. Conversely, if the communication system immediately changes a buffer unit to store user data based on the changing of a cell association of the mobile terminal, the user data remaining in a previous buffer unit corresponding to a cell previously associated with the mobile terminal is discarded. Accordingly, the discarded user data needs to be retransmitted to the current buffer unit corresponding to a cell currently associated with the mobile terminal.
In order to solve the above-noted problems of related arts and to provide improved communication systems, a first object of the present invention is to provide a base station in which a retransmission of user data destined for a mobile terminal can be suppressed by preventing the user data from being lost due to the changing of cells with which a mobile terminal associates.